1. Field of the Invention
The present invention generally relates to charged particle beam projection lithography tools and, more particularly, to electron beam (e-beam) projection masks or reticles.
2. Description of the Prior Art
Numerous lithography techniques are known and are in widespread use for manufacture of integrated circuit devices, in particular. Essentially, such lithography processes define potentially very minute areas and shapes on a surface through selective exposure and removal of portions of a layer of resist to expose areas of a surface for further processing by, for example, etching, implantation and/or deposition. Exposure has generally been accomplished in the past by use of radiant energy such as visible wavelength light.
There is a strong incentive in the manufacture of integrated circuits to increase integration density to the greatest possible degree consistent with acceptable manufacturing yields. Device arrays of increased density provide increased performance since signal propagation time is reduced with reduced connection length and capacitance. Further, increased device density on a chip allows greater chip functionality as well as greater numbers of devices which can be manufactured on a chip of a given area. Accordingly, increased integration density reduces manufacturing costs by maximizing the number of devices (or the number of chips of a particular functionality) which can be formed in the course of processing a single wafer of a given size. That is, for a given processing schedule, the cost of processing each wafer (including amortization and maintenance of the tools and processing reactor vessels) is substantially constant regardless of the number of devices or chips simultaneously formed.
While electromagnetic radiation (EMR) has been widely used in the past for exposure of lithographic resists, resolution is limited by the wavelength of the radiation employed. To produce very small feature sizes for high integration density, deep ultra-violet (DUV), extreme ultra-violet (EUV) and x-rays have been investigated for exposure of lithographic resists. However, it is generally considered that EMR is not suitable for minimum feature size regimes at or below one-tenth micron. To resolve such minimum feature sizes, charged particle beams are generally believed to be required. Electron beams are generally preferred since the much lower mass of electrons, relative to ions allows the beam to be readily controlled with relatively lower power requirements of the exposure tool optical elements.
However, to be economically viable for production of integrated circuits, electron beam tools must have throughput which is comparable to EMR lithography exposure tools. To achieve such throughput, exposure must be accomplished over relatively large sub-fields often containing one million pixels or more by projection of a pattern formed on a reticle or mask placed in the electron beam path. The masks must be formed with extremely high dimensional accuracy so that the sub-field patterns can be accurately abutted or "stitched" together into a larger continuous pattern.
Masks for electron beam projection rely on scattering of electrons predominantly by one of two basic mechanisms and are thus of one of two basic physical types. Aperture or stencil masks have a thin membrane, generally of silicon, corresponding to sub-field areas which are respectively patterned by etching of holes therein. While the membrane of aperture masks is thin enough for electrons to penetrate, the electrons penetrating the membrane are scattered sufficiently to be intercepted by a contrast aperture before reaching the target while electrons penetrating an aperture reach the target unimpeded.
The other type of mask has a continuous membrane of a material, such as silicon nitride, which is much thinner than the membrane of an aperture mask such that electrons can readily penetrate the membrane without significant scattering. The membrane is patterned with a pattern of material such as tantalum formed thereon which is capable of strongly scattering electrons which are incident on the pattern of material. A separate contrast aperture is generally used with this latter type of scattering mask, as well, to collect strongly scattered electrons and prevent them from reaching the target.
Both types of mask are generally constructed from a relatively thick sheet of material which is reduced in thickness (e.g. by etching) in the sub-field areas to form mask membrane areas. The remaining relatively thick material between the sub-field membrane areas thus defines a cross-hatch pattern or grillage of thicker material; portions of which are referred to as struts. The grillage pattern provides mechanical support to the membrane areas and a path to thermal ground for the few electrons absorbed in the thin membrane.
The beam is sized, shaped and positioned to fill the sub-field membrane areas with little or no electron impingement on the struts. As is known, electron absorption by any portion of the mask will cause heating which, in turn, increases the risk of thermal distortion of the mask. Thermal cycling and large temperature excursions can also cause persistent distortions of the mask and/or the sub-field membrane areas thereof. Additionally, while masks are made with extremely high precision, distortions may occur in the mask, as fabricated, when material is removed or deposited in the desired pattern and internal stresses in the mask are thus altered. Thermal cycling can also alter internal stresses in the mask which may result in or contribute to mask distortion.
The quality of the projection mask is critical to implementation of projection e-beam lithography. It can be readily understood that the distortion of the mask must be held to a small fraction of the feature size or pitch (e.g. in the mask itself or referred to the target plane since demagrification is usually employed) if exposures of sub-fields are to stitch together properly. Distortions due to temperature, however, are superimposed on any distortions in the mask, as fabricated, and persistent distortions which may occur through use of the mask in the e-beam exposure tool. Therefore, it is necessary to evaluate the mask periodically to determine the extent of persistent distortion which may have occurred during use and to evaluate the quality of the mask periodically over its useful lifetime.
It is known to provide registration and measurement marks on masks in regions which correspond to the kerf between chips on a wafer. This location is chosen because measurement marks on known exposure masks also results in exposure of resist on the wafer corresponding to the measurement mark. However, when sub-field exposures are made and stitched together, such exposure would overlap another sub-field and thus cannot be readily accommodated without compromising the design and layout of the chip and consuming significant chip space. By the same token, measurement marks provided only at the perimeter of the mask do not reveal distortions which may occur in sub-fields and which may partially compensate each other over the dimensions of the entire mask.
Accordingly, prior mask structures and designs do not provide measurement marks which avoid printing of the measurement mark on the wafer or provide for detection, much less allowing optical correction or compensation, of local (e.g. sub-field) distortion of the mask. Therefore, known mask structures and designs do not allow initial assessment or evaluation and/or monitoring of the quality of the mask, which is critical to high throughput e-beam projection lithography at small feature size and fine pitch.